I think the model of the Directive on ferry delays (1177) is much better (and clearer). Compensation here is based on a percentage of the ticket price (and that % rises with the length of the delay); food, accom and comms also have to be provided.
EU261 does have a proportional-rebate clause for involuntary downgrades.
I believe it wasn't adopted for the delay / cancellation conditions because it would tempt airlines into favouring one group of passengers over another ( e.g. if the business-class-fares London City flight went tech, dump the tourists off the cheap-fares Ibiza aircraft and use it instead. ).